[PATCH 5/6] slab: validate slab before using it in alloc_single_from_partial()

Vlastimil Babka
2025-09-11T17:02:47Z

We touch slab->freelist and slab->inuse before checking the slab pointer
is actually sane. Do that validation first, which will be safer. We can
thus also remove the check from alloc_debug_processing().

This adds a new "s->flags & SLAB_CONSISTENCY_CHECKS" test but
alloc_single_from_partial() is only called for caches with debugging
enabled so it's acceptable.

In alloc_single_from_new_slab() we just created the struct slab and call
alloc_debug_processing() to mainly set up redzones, tracking etc, while
not really expecting the consistency checks to fail. Thus don't validate
it there.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/slub.c | 17 ++++++++---------
1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 909c71372a2f542b6e0d67c12ea683133b246b66..93df6e82af37c798c3fa5574c9d825f0f4a83013 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1651,11 +1651,6 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
struct slab *slab, void *object, int orig_size)
{
if (s->flags & SLAB_CONSISTENCY_CHECKS) {
- if (!validate_slab_ptr(slab)) {
- slab_err(s, slab, "Not a valid slab page");
- return false;
- }
-
if (!alloc_consistency_checks(s, slab, object))
goto bad;
}
@@ -2825,15 +2820,19 @@ static void *alloc_single_from_partial(struct kmem_cache *s,

lockdep_assert_held(&n->list_lock);

+ if (s->flags & SLAB_CONSISTENCY_CHECKS) {
+ if (!validate_slab_ptr(slab)) {
+ slab_err(s, slab, "Not a valid slab page");
+ return NULL;
+ }
+ }
+
object = slab->freelist;
slab->freelist = get_freepointer(s, object);
slab->inuse++;

- if (!alloc_debug_processing(s, slab, object, orig_size)) {
- if (validate_slab_ptr(slab))
- remove_partial(n, slab);
+ if (!alloc_debug_processing(s, slab, object, orig_size))
return NULL;
- }

if (slab->inuse == slab->objects) {
remove_partial(n, slab);

--
2.51.0